1258 THE STRUCTURE OF EVOLUTIONARY THEORY
four evangelists at the eastern end (where the sun rises upon novelty), contrasted with
the four great Old Testament prophets to the west (where the sun sets on ancient
ways).
Three major reasons for the centrality of spandrels, and therefore
of nonadaptation, in evolutionary theory
I therefore find the concept of spandrels, or features of nonadaptive origin as
structural byproducts or side consequences of other architectural decisions, to be both
coherently definable and eminently testable. The importance of spandrels in
evolutionary biology must then rest on two further attributes: (1) their engagement
with conventional theory in a challenging way that suggests potentially important
changes or expansions in our general understanding of evolution; and (2), their
establishment as sufficiently common to constitute a high percentage of biologically
and evolutionarily relevant traits of organisms and other biological individuals.
I think that spandrels pass the first test in a robust manner, for their existence at
high relative frequency (the claim of the second test) would challenge a key
procedure of the adaptationist program that has long served as the day-to-day
working methodology of Darwinian biologists engaged in the explanation of
particulars. At the most basic level, we simply cannot gain an adequate evolutionary
explanation for a trait by elucidating, however elegantly, however experimentally,
and however quantitatively, its contribution to the fitness of the organisms or
populations in which it now resides. Purely adaptationist analysis therefore cannot
resolve history for two major reasons:
- Through the principle of quirky functional shift, and Nietzsche's discordance
between reasons for current utility and sources of historical origin, our understanding
of how a current trait works cannot elucidate its mode of origin—an ineluctably and
logically central task of evolutionary explanation, and one of the most interesting
questions that any historical science can pose. - Adding insult to injury, even the most sophisticated documentation of
adaptive value in a current feature gives us no right to assert similar adaptational
control over its past states—even admitting the principle of quirky functional shift,
and the possibility of strikingly different past usages, with current functions emerging
as exaptations. Rather, the principle of spandrels suggests that a high percentage of
traits now contributing in important ways to fitness arose for no adaptive reason at
all, but rather as automatic side consequences of other forces (usually selection on
other aspects of the organism to be sure, but with no direct selection on the trait under
study). The adaptationist program cannot provide a full accounting of evolutionary
change if a high percentage of traits originated as nonadaptive spandrels.
We must then pose the second question about the relative frequency of
spandrels. If rare, they remain conceptually interesting, but minor in actual
importance for the evolutionary understanding of particular lineages—the bread and
butter of daily practice in our science. My broader case for the high frequency, indeed
for the near ubiquity, of spandrels occupies the last section